Modafinil is a pharmacologically unique wake-promoting agent whose exact mechanism of action is unknown. This is a doubleblind, placebo-controlled, randomized, parallel-group study of the effect of 4 weeks of modafinil on excessive daytime sleepiness in patients with obstructive sleep apnea who are being treated with continuous positive airway pressure (CPAP). Patients must have been using CPAP for at least 2 months prior to study entry. After a 21 day assessment period of CPAP use, patients will be randomized to receive either modafinil or placebo for four week. Patients will receive modafinil 200mg/d for one week and will then receive 400 mg/d for the rest of the study. A 12 week extension has been added.